1. Field of the Invention
The present invention relates to a semiconductor single-crystal growth system which grows single crystals using a crucible with a double-walled structure, and particularly to a system which includes suppression of the stagnation of CO gas near a melt (hereunder referred to as a "starting melt"), which leads to higher quality single crystals and enhanced manufacturing yields.
2. Description of the Related Art
The Czochralski method (CZ method) is widely used as a method for manufacturing single crystals. This method pulls a growing crystal up from a melt in a crucible.
Attempts to obtain silicon single crystals doped with dopants such as B, P, Sb or the like by the CZ method, have been faced with the problem of non-uniform dopant concentration along the length of the crystal. Since these dopants have segregation coefficients other than 1, the single crystals grown with the CZ method only have the desired quality in certain sections.
As solutions to this problem, Japanese Unexamined Patent Application Disclosure SHO 49-10664 and U.S. Pat. No. 4,352,784, for example, disclose techniques for increasing the yields. These techniques use the so-called double-walled crucible, composed of an outer crucible and an inner crucible placed therein, which are partitioned with an annular diaphragm equipped with a communicating hole drilled therethrough for passage of a starting melt. By using the two melts, the dopant concentration in the inner crucible can be stabilized by supplying some of the melt from the outer crucible. These methods help to establish a more uniform dopant concentration across the growing single crystal. FIG. 8 is a schematic view of such a structure.
FIG. 8 shows a single-crystal growth system comprising a furnace 1 which includes a quartz crucible 2 resting on a vertically movable and rotatable lower shaft 8 which extends from a drive unit (not shown) placed outside the furnace 1 and penetrates through the bottom of the furnace 1. The crucible 2 is a double-walled crucible which comprises an outer crucible 2A receiving a cylindrical inner crucible 2B therein, with the outer surface protected by an encircling graphite susceptor 3. The inner crucible 2B is separated from the outer crucible 2A with an annular partition wall equipped with a communicating hole drilled therethrough for passage of a starting melt. In addition, there are provided a cylindrical heater 4 encircling the crucible 2 with a certain spacing, and a heat insulating mold 5 positioned outside the heater 4. Argon gas is supplied to the furnace 1 through a gas inlet 6 and discharged via a gas outlet 7 together with impurities produced from the starting melt Y.
Use of this system can lead to non-uniform carbon concentrations, reducing the yield. Manufacturing silicon single crystals with the above-described system equipped with a double-walled crucible results in a single crystal with a gradual increase in the carbon concentration in the direction from the top to the tail of its growth. Parts of the crystals have carbon concentrations higher than that suitable for use as semiconductor devices, thus decreasing the yields of single crystals.
The carbon contamination originates from a variety of graphite parts (e.g., the heater 4, heat insulating mold 5, susceptor 3, support for the inner crucible 2B, etc.). First, the starting melt Y reacts with the quartz of the crucible 2 to produce silicon monoxide (SiO) gas. This SiO in converted to CO on the surfaces of the graphite parts at high temperatures, according to the following equation: EQU SiO+2C.fwdarw.SiC+CO
Since carbon contamination is more severe with double-walled crucibles than with the usual single-walled crucibles, double-walled crucibles must have some mechanism which facilitates the dissolution of the evolved CO into the starting melt Y.
The inventors of the present application have conducted a close examination of the behavior of CO gas in double-walled crucible furnaces. As shown in FIG. 8, they have found that the gas stagnates in the space between the inner crucible 2B and the outer crucible 2A, immediately above the starting melt Y, for a relatively long period to cause an increase in the CO concentration of the gas, and to allow continuous contacting of the CO with the melt Y. This results in the ready dissolution of the CO into the melt Y, increasing the carbon concentration of the melt and eventually of the single crystal T.
Based on these findings, the present inventors suggested a structure, as shown in FIG. 9, for example, which comprises a double-walled crucible 2 and a cylindrical gas-flow guide cylinder 9 positioned above, and coaxially with, the crucible 2, with the lower end of the gas-flow guide cylinder 9 placed in the space between the inner crucible 2B and the outer crucible 2A, and with spacings between the lower end, inner crucible, outer crucible and starting melt, respectively. (Japanese Unexamined Patent Application Disclosure HEI 2-116697). With this structure, the flow of the gas in the furnace was indeed improved, and high quality single crystals were obtained, as compared with the previous growth systems based on double-walled crucibles; nevertheless, the flow guide effect was not satisfactory, leaving much to be desired.